(1) Field of the Invention
The present invention relates to an optical switch changeover controlling technique in conducting a changeover of an optical path by a spatial optical switch comprising a plurality of optical switch elements arranged therein, and particularly to an optical switch changeover controlling method, an optical node device and an optical switch system for realizing an uninterrupted changeover of an optical path.
(2) Related Art
Recently increased information capacities, variation and the like require a flexible and reliable construction of a network as well as an increased capacity of a transmission system. As one way to realize them, there has been demanded a construction of an optical network based on a wavelength division multiplexing (WDM) technique. In constructing such a network, important roles will be played by an optical-switch-adopting optical node device, such as: an optical cross-connect device for setting a bypass route, e.g., when changing over an optical path or when a fault occurs in a transmission path; an optical add drop multiplexer (OADM) for adding/dropping optical signals; and an optical protection device for conducting a recovery from a fault in an optical network.
FIG. 18 is a conceptual diagram for explaining an optical switch changeover controlling method utilizing conventional optical node devices. There is shown an example of a procedure for changing over an optical path from a working ray path to a protective ray path at the time of occurrence of a fault, in which FIG. 18A shows an initial state, FIG. 18B shows a state where a fault occurs, FIG. 18C shows a state where a path release is conducted, and FIG. 18D shows a state where the recovery from the fault has been completed.
In the initial state of FIG. 18A, two optical node devices 1A, 1B are interconnected via a working ray path 2W and a protective ray path 2P. The working ray path 2W is input with a client optical signal sc from a client (terminal equipment) 3A connected to the optical node device 1A, and the protective ray path 2P is input with a PCA optical signal sp from a PCA (Protect Channel Access) device 4A connected to the optical node device 1A.
When a fault such as a disconnection occurs in the working ray path 2W as shown in FIG. 18B, the client optical signal sc being connected to the working ray path 2W is to be changed over to the protective ray path 2P. Concretely, the connection of the PCA optical signal sp having been connected to the protective ray path 2P is once released (xe2x80x9cpath releasexe2x80x9d) as shown in FIG. 18C, followed by a reconnection of the client optical signal sc to the protective ray path 2P as shown in FIG. 18D to thereby conduct a changeover from the working ray path 2W to the protective ray path 2P at the time of occurrence of the fault.
There will be now briefly explained a changeover operation of optical switches provided in the optical node devices 1A, 1B.
As a typical optical switch to be provided in each of the optical node devices 1A, 1B, there is used an Nxc3x97N spatial optical switch, for example, which is constituted of matrix-arranged N2 units of 2xc3x972 optical switch elements (in which N is the number of lines to be changed over at the node), where each 2xc3x972 optical switch element has two inputs and two outputs cooperatively changeable into one of a parallel (bar) state and an interlaced (cross) state.
FIG. 19 is a diagram showing an example of a 2xc3x972 spatial optical switch (i.e., N=2) in the transmission side optical node device 1A. There is shown a procedure for changing over, the path of the client optical signal sc connected from an input terminal #1i to an output terminal #1o of the optical node device 1A, to a path from the input terminal #1i to an output terminal #2o.
In a path setting initial state shown in FIG. 19A, an optical path for transmitting the client optical signal sc from the input terminal #1i to the output terminal #1o as shown by a solid line arrow, and an optical path for transmitting the PCA optical signal sp from an input terminal #2i to the output terminal #2o as shown by a dotted line arrow are set. At this time, a 2xc3x972 optical switch element S11 at the intersection point between the input terminal #1i and output terminal #1o, and a 2xc3x972 optical switch element S22 at the intersection point between the input terminal #2i and output terminal #2o are brought into parallel states (ON states), respectively.
Note, xe2x80x9ca 2xc3x972 optical switch element at an intersection point between an input terminal #xi and an output terminal #yoxe2x80x9d means such a 2xc3x972 optical switch element in the parallel state: when all 2xc3x972 optical switch elements within a spatial optical switch are once turned into interlaced states (OFF states) and one of the 2xc3x972 optical switch elements is then changed over to a parallel state (ON state) to thereby set an optical path directed from the input terminal #xi to the output terminal #yo.
FIG. 19B shows a state where the already connected two optical paths are released before conducting a changeover of a path connection, in which all the optical switch elements S11, S12, S21, S22 are in the interlaced states (OFF states) (path released states).
FIG. 19C shows a path reconnection state where the 2xc3x972 optical switch element S12 at the intersection point between the input terminal #1i and the output terminal #2o is brought into the parallel state (ON state) so as to set an optical path from the input terminal #1i to the output terminal #2o to thereby connect the client optical signal sc to the protective ray path 2P.
In the conventional optical node device as described above, there is conducted a consecutive procedure including optical path setting (initial state), optical path release and optical path reconnection, when conducting a connection changeover of an optical signal. Thus, an optical output power (optical output power to the protective ray path 2P) of the output terminal #2o of the optical node device 1A is interrupted in the course of the changeover of the optical path, as shown in FIG. 20. Concretely, this interruption in the optical output power continues over a period of time from the optical path release up to the optical path reconnection, and the interruption period Txe2x80x2 can be represented by the following equation (1):
Txe2x80x2=Tf+Toff+Trxe2x80x83xe2x80x83(1)
wherein Tf is a falling time of the 2xc3x972 optical switch element, Toff is a changeover controlling time (time-lag up to the optical path re-setting), and Tr is a rise time of the 2xc3x972 optical switch element.
As such, the conventional optical node device as described above has a possibility to cause a deterioration of optical signal transmission characteristics or a failure of the device, due to the interruption in the optical output power at the time of changeover of the optical path. Namely, such as when an optical amplifier is arranged on the latter stage side of an optical switch within an optical node device or is arranged within an optical transmission path interconnecting optical node devices, an interruption in optical output power at the time of changeover of an optical path will cause an optical surge in the optical amplifier. This results in a problem of a possibility to cause a deterioration of optical signal transmission characteristics or a failure of the device. Further, the aforementioned optical switch changeover controlling method in the conventional optical node device also has a defect of the time-lag up to the reconnection of the optical path.
The present invention has been carried out in view of the conventional problems as described above, and it is therefore an object of the present invention to provide an optical switch changeover controlling method, an optical node device and an optical switch system, capable of avoiding an interruption in optical output power at the time of changeover of an optical path.
To achieve the above object, an optical switch changeover controlling method according to the present invention, for using a spatial optical switch which is provided with a plurality of optical switch elements arranged between a plurality of input terminals and a plurality of output terminals thereof, each of the plurality of optical switch elements being controllable to change over a connection between two input ports and two output ports into one of a parallel state and an interlaced state; and having such a characteristic that a power of optical signal to be output from each of the output ports is continuously changed over from a power of optical signal to be input to one of the two input ports, to a power of optical signal to be input to the other of the two input ports, to thereby change over setting of optical paths interconnecting between the plurality of input terminals and the plurality of output terminals of the spatial optical switch, comprises:
when a first optical path directed from a first input terminal to a first output terminal of the spatial optical switch is changed over to a second optical path directed from a second input terminal to the first output terminal,
an optical path re-setting step for initiating, re-setting of the respective optical switch elements for forming the second optical path, while keeping the setting of the respective optical switch elements for forming the first optical path, to prepare a state where, for one optical switch element participating in forming both of the first optical path and the second optical path, one of the two input ports is input with an optical signal from the first input terminal and the other of the two input terminals is input with an optical signal from the second input terminal; and
an optical path reconnection step for changing over the connection state of the one optical switch element participating in forming both of the first optical path and the second optical path, set in the optical path re-setting step, to the other connection state to thereby release the first optical path and establish the connection of the second optical path.
According to such an optical switch changeover controlling method, when changing over the connection setting of the optical path, there can be temporarily realized, by the path re-setting step, a state where the first optical path before the changeover and the second optical path after the changeover are simultaneously set. This enables an uninterrupted changeover of the optical path, as well as suppression of occurrence of optical surge in optical amplifiers. In this way, it becomes possible to achieve stable transmission characteristics of optical signal, and to reduce the frequency of device failures.
The optical switch changeover controlling method according to the present invention as described above can be applied to an optical node device constituted using a spatial optical switch and to an optical switch system, for example. Further, it is also effective to construct an optical network making use of a plurality of optical node devices to which the present invention is applied.